This invention relates to data compression method of a perfect or partial reproduction type which can effective compress dot image data.
The compression technology for pseudo-halftone images which represent half tones with binary data is widely used for transmitting data such as half tone image or newspaper iamges by a facsimile. This is because they cannot be directly transmitted or stored in a memory effectively as the volume of image data is extremely large. In data transmission by the facsimile, data compression technology has been used to transmit data at a high efficiency by an MH method (one dimensional compression: Modified Huffman Coding) or an MR method (two dimensional compression: Modified-READ) in the prior art. These conventional methods, however, are not appropriate as the technique to compress the dot image data. If they are applied to the pseudo-halftone images, an array of the image data should be re-arrange in the order of the threshold values in a dither matrix to produce longer runs so as to transmit character images more effectively, but it is not easy to select the method to change the data array which reduces the number of small runs of about one to three bits.
There has been proposed a prediction encoding method which considered not only the state of leading reference pixels of m number but the position of the current pixel within the dither matrix. The method is also defective in that it is difficult to implement as a practical device if the number m of states is to be increased. Even if the aforementioned conventional MH method or MR method is applied as it is, the compression rate is low.